Cars 101: How much power does a car really make?Posted: July 3, 2011
Horsepower ratings: how much power does your car have? Well, theres a few different answers….
Dynamometer (Dyno): device used to find the amount of power being made by a spinning force. There are both engine dynos (to measure a motor’s power) and chassis dynos (to measure the power of an entire car as a whole)
Brake Horsepower (BHP): this is the amount of power the engine alone makes. Manufacturers us this in their ratings and advertising because they only rate the power of the motors they produce in a lab overseen by the Society of Automotive Engineers.
Wheel Horsepower (WHP): This is the amount of power the car as a whole makes where the tires meet the ground. It is usually a certain percentage less than the BHP because energy is lost in the transmission and drive train. Percentage losses are roughly as follows: FWD (10-15%), RWD (15-20%), and AWD (20-30%). Each type of car has its own amount of loss depending on what materials are used in the drive train, and specific car will vary from others of the same type to some degree in general.
Here is a video of how a chassis dynamometer is used to measure a cars power output. In this video a heavily modded Corvette Z06 makes 904 horsepower at the rear wheels, truly awesome:
In essence the horsepower number that the manufacturer tells you a car has is only what the engine itself produces, they do not rate the car as a whole for various reasons relating to consistency and marketability. In reality the only horsepower number that matters for realistic speed calculation is power at the drive wheels in the pavement. You could have an awd car with 400bhp and a fwd car with 380bhp as rated by the manufacturer, however in reality the fwd car may be more powerful than the awd car due to more drivetrain loss from driving all four wheels.